Now to prove Binet’s theorem using the above lemma and the definition of the Fibonacci
series given in Definition 2.1.
Theorem 3.2. For all n ∈ N, Fn = B(n) = (1+√
5)n−(1−
√
5)n
2n
√
5
.
Proof. By application of the extended principal of mathematical induction on n. Let us
begin by defining the statement S(n) to be Fn = B(n) = (1+√
5)n−(1−
√
5)n
2n
√
5
. We will show that
the truth set of S(n) is equal to N by first proving two base cases, n = 0 and n = 1.
Case 1 (Base Case n = 0). S(0) is shown directly: